Hi, we're new to machining but we have a problem with our parting tools. Even new ones tend to (a lot but not always) part off leaving a dish or a dome on the working faces. The tool is set perpendicular to the lathe axis, the tool is centred, and the cross slide is locked during parting. Any ideas why this happens?

Many thanks for any insights you can offer.

Chris

Is the carriage locked with a shop made lock? The concave face is because the carriage is flexing away from the chuck. Use a carriage lock when facing and parting off.There are several versions that are easy to make that you can download. Google " carriage lock". I made a simple clamping device that clamps the carriage on the chuck side of the saddle. A 1/4" thick piece of steel was cut to fit between the underside of the bed ways. Then drilled and tapped for a 1/4-20 hex head bolt. The two tapped holes for the follow rest were used to fix a steel bar across the cross feed saddle. A clearance hole for the bolt was bored. I also fixed a pin that went thru this bar and into a hole in the clamping bar.This was added because originally the clamp some times turned a bit when tightening it and would bind on the underside of the bed ways.Tighten the hex head bolt and the carriage will not move. I use a nut driver that fits the bolt head instead of making a lever of some sort.Clamping on the side towards the chuck as I do prevents the cutter from lifting.If you prefer to clamp on the other side you may get some tendency for the cutter to lift slightly.

mike

October 18, 20116:49 AM

ChrisXenon

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This is for "Paddleduck John" - also posted under his BLOG:

Hello John and many thanks for the time you have given us, which of course, is in addition to the time you took to make the plans and the BLOG in the first place.

Where should I have posted these comments? I’ll put them here, and maybe also in a forum.

We made the jig exactly as you described, and the plate is really tough “hot rolled” thick steel.

We do face the webs before parting, so there is one nice square face, and one groovy dishy one, and we initially used the jig in the way you describe – “upside down” – peg down between the jaws of the post drill vice. Then manually guided the drill into the holes in the thick steel plate of the jig, and down into the web.

Which way up should the web go? I guessed good side down – where it will sit on the drill vice squarely. We didn’t face the bad side as we couldn’t see a way to hold it. Your pot chuck idea is new to us, and perhaps we could make one, and use it to make new webs, but I don’t understand what the benefit would be beyond cosmetic. (that’s just a literal statement of my ignorance – not a contradiction of your advice!). In addition, since parting is such a noisy and stressful process, the prospect of putting my head near the lather through four more of them is very daunting.

Later on when our post drill-and-jig-upside-down method proved inadequate, we dumped the jig, and used the milling machine'’ table to drill accurately.

We are slitting with a hacksaw, and after the web is shaped, and I have to say it’s a carve-up. Not straight, not central, and not consistent between webs. I don’t know how we would do better. But we thought this wasn’t important – what seems to be important is that we make a gap to allow the straight, parallel sides of the holes to pinch on the shafts, thereby holding them in correct registration.

You say: “Hopefully, by using the jig, all holes should now be square.”

The central issue is that, simply, they ain’t.
And we don’t know why nor how to fix it.

We are using steel as recommended. We could use brass, but since it’s softer, I would have thought it’s more prone to allow deviations from true. The parting off could be more fun though…

Your offer of a visit is very kind indeed John, thank you. However, we live 150 miles away which is a six hour round trip, not including any “shed time”.

Perhaps the answer lies in tramming – I’d never heard of that phrase before. We did check that the quill was true – or seemed to be – but thinking about it, it was very crude – putting a rod in the chuck, and assessing its squareness visually, against a set square placed on the bed.

Another engineer suggested making the crank with the shaft all in one piece, glueing iti n place, then removing the unwanted parts of the shaft. I can see how this would true up the shaft segments, but the idea of relying on glue to hold a shaft in alignment in a hole which isn’t true seems very dodgy, and also, removing the unwanted shaft segments would seem far from trivial. In any case, it would not help us with trueing up the big end segments of the crankshaft.

We’ve also considered using square rods in place of the webs, and doing away with some of the bearing blocks to give us more room for a more chunky design.

Also, as I mentioned, cross stitch is looking very attractive.

If you have anything to add, John, we’d love to hear it, but if you’re out of ideas, then elt me thank you once again for all the time you have given us.

Best Wishes,
Chris

July 7, 20114:34 AM

ChrisXenon

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Well, now we have tried it and it works. Just in case anyone else is having the same problem.

I spoke to Axminster, UK dealers for this machine. The spindle lock works through a pinching action between two profiled blocks - one behind and one in front and both slightly to the right of the spindle. When you turn the handle, they pinch together, biting on the spindle, preventing it from moving.

Apparently, it's quite common that the threaded shaft is a little too long to get enough bite. Placing a couple of washers on the front of the shaft just bejhind the handle, effectively shortens this shatfs and allows more bite. We haven't tried it yet, but it seems to make sense.

Chris

June 15, 20116:09 AM

ChrisXenon

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I'm supposed to know that LMS is the name of a store on the other side of the world!?

But seriously, thanks.

However, I cannot find the exploded diagram you mention there. A search for SIEG X" finds nothing, and a search for SIEG finds, apparently, everything they sell on the website. There is a mini mill X2 but it is not the Sieg.

June 14, 20111:29 PM

Rob

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LMS is LittleMachineShop.com. A well-known company that sells, among other things, parts and accessories for nearly all the Asian made lathes and mills that are popular among home machinists.

The pin fits into a slot in the spindle. If the lock is pushed in while the spindle is turning, it shears off the end of the pin. You'll probably have to make a replacement pin, and possibly clean up any burrs on the top of the spindle itself. LMS has exploded parts diagrams if you need them.

Our mill has a "spindle locking lever" which doesn't lock the spindle. We can turn it indefinitely, and it sort of bites but doesn't lock the spindle. When we bought the machine second-hand, the pin for this lever had fallen out, and we wonder if something fell out which we didn't replace - a ball bearing, perhaps. Anyway, please tell us how this spindle SHOULD operate, and if you have ny ideas about how to diagnose and fix it, those would be appreciated too. Right now, everything we mill is wrong in some way, and it's getting a bit tiresome.

Thanks,

Chris

June 8, 201112:50 AM

ajejeje

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The comments and conversations are great, i learned a lot from you guys.

concrete floor

May 29, 20119:47 PM

Rob

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I'm not familiar with your lathe but I found its specs and it's a lot bigger and therefore probably much more rigid that the "mini-lathe," which many people won't even try to part off on.

I've heard experienced machinists recommend having your blade a little bit above the center line and I've found that to helpful. You definitely don't want it below center because the work piece will want to climb the blade. Which may cause it to twist more.

I've found it helps if you don't extend the compound any more than it has to be. That keeps the blade closer to the middle of the carriage where it's likely to be more rigid, instead of hanging way out on the end of long arm the work piece will have more leverage on.

I also starting getting much better results when I bought the best quality parting blade I could find. It's an American made HSS "T" shaped blade. You might also try slowing down your RPMs and feed and see if that helps. Keep it well lubed too.

For what it's worth. I almost always have a noticeable concave/convex surface when I part on my 7x12 mini-lathe. I'm getting much better results with my much more rigid 8x14, but still not quite perfect. The only lathe I've gotten perfectly flat part-offs on was a industrial quality Hardinge lathe I got to use while taking a class. That lathe of course was very rigid and it had a very rigid part off tool that used carbide inserts. (I'm a mostly self-taught amateur so I don't know if my results are typical).

Your lathe is similar in size to what we would call a 9x20 here in the states. If none of this helps then you might want to ask for advice on one of the discussion groups for that lathe, such as the Lathemaster discussion group.

This seems like a real mystery. I'd really like to know if you find the cause/solution.

Rob

May 29, 20112:22 PM

ChrisXenon

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Thanks for your help, gents.

Rob, it's hugely noticeable. A parted off piece wobbles like a weeble (and it MAY fall down).

The lathe is a chester/chinese one.It's in my brother's shed so I don't lknpw the model, but looking at pictures it's very like the the DB10G. Yes, the carriage is properly locked, the gibs are adjusted to keep things tight and there's no play. The blades (we've had a few) are generic un-branded things we've picked up a tool fairs etc. 2mm wide. We cut as close to the chuck as possible and the blade is as far into the tool as the width of the iece permits, which is up to 25mm.

Your question about how far out the thing is cranked out from the post is a good onw qwhicxh I don't hvae the answer to. We'll look into that soon - thanks.

Mikey, thanks. It's on my list to look into rear-mounted parting tools. Maybe we need to stop trying to ignore this misery and spend an entire afternoon sorting it out.

Thanks again,

Chris

May 29, 201112:55 AM

Mikey

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I agree with Rob that the most likely cause of your problem is probably flexion of the parting tool, followed closely by a misaligned tool, followed by rotation of the tool holder holding the parting tool. You want to extend the parting tool only enough to make the cut to center and be sure you align the tool to the chuck's face so it is at a right angle to the work. You can add a brace to be sure the tool holder does not rotate, then part as close the chuck as possible. Be sure the tool is sharp and use cutting oil and a positive feed.

If you are using a small lathe then look into a rear-mounted parting tool holder. It is a vastly more rigid setup than a front mounted holder in a quick change tool post.

If you can answer Rob's questions it would help us to be more helpful.

Mikey

May 28, 201111:34 PM

Rob

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It sounds like you're doing everything right. But something must be wrong so I have these questions:

Just how bad is the convex/concave surface you're getting? Pretty noticeable or just barely?

What kind of lathe do you have? (I'm not going to be surprised at all if you tell me you have a mini-lathe).

Are you sure the carriage is locked and can't move at all?

Are the gibs adjusted properly? Is there any play at all in the compound?

What kind/quality blade do you have? Is it positioned close to the compound or cranked out so it's on the end of a long arm?

What size and kind of material(s) are you working with?

May 28, 20113:24 PM

ChrisXenon

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A forum search found nothing on this, so here goes.

Hi, we're new to machining but we have a problem with our parting tools. Even new ones tend to (a lot but not always) part off leaving a dish or a dome on the working faces. The tool is set perpendicular to the lathe axis, the tool is centred, and the cross slide is locked during parting. Any ideas why this happens?